Concrete is a composite material consisting of a mineral-based hydraulic binder which acts to adhere mineral particulates together in a solid mass; those particulates may consist of coarse aggregate (rock or gravel), fine aggregate (natural sand or crushed fines), and/or unhydrated or unreacted cement. Concrete dates back at least to Roman times. The invention of concrete allowed the Romans to construct building designs, such as arches, vaults and domes, which would not have been possible without the use of concrete. Roman concrete, or opus caementicium, was made from a hydraulic mortar and aggregate or pumice. The hydraulic mortar was made from quicklime, gypsum or pozzolana and combinations thereof. Quicklime, also known as burnt lime, is calcium oxide; gypsum is calcium sulfate dihydrate and pozzolana is a fine, sandy volcanic ash (with properties that were first discovered in Pozzuoli, Italy). By using concrete, the Romans were able to build arches, vaults and other structures that were not possible to build before. However the concrete made with volcanic ash as the pozzolanic agent was slow to set and gain strength. Most likely the concrete was build up in multiple layers on forms that had to stay in place for a very long time. Although the concrete was slow to set and gain strength, over along periods of time it achieved great strength and was extremely durable. There are still Roman concrete structures standing today as a testimony to the quality of the concrete produced over 2000 years ago.
Due to the slow setting and great length of time that it took for the early concrete to gain strength and forms to be removed, it never gained broad acceptance. In fact, it appears that it ceased to be used after the fall of the Roman Empire. Stone and clay brick masonry became the preferred method of construction for most of human history.
In the late 1700's different types of Roman Cements were patented and in 1824 Joseph Aspin filed a patent for the method of making what is known as portland cement. The new manufactured cement resulted in faster hardening cement with a higher compressive strength. During the 19th century there were many improvements made to the process of manufacture of portland cement. The concrete made with the portland cement allowed the concrete to set fast and to gain strength sufficient to support itself in a short amount of time. Therefore, the concrete forms could be removed quickly and construction schedules could be shortened.
Modern concrete is composed of one or more: hydraulic cements, coarse aggregates, fine aggregates and of course water. Optionally, modern concrete can include other supplementary cementitious materials, inert fillers, property modifying chemical admixtures and coloring agents. The hydraulic cement is typically portland cement. Other cementitious materials include fly ash, slag cement and other natural pozzolanic materials. Mortars are also made from cementitious material, aggregate, water and optionally lime.
Portland cement is the most commonly used hydraulic cement in use around the world today. Portland cement is typically made from limestone, as well as clay, sand, or shale, among other raw materials. The raw materials for portland cement production are proportioned to obtain a desired mixture of minerals containing calcium oxide, silicon oxide, aluminum oxide, ferric oxide, and magnesium oxide. The raw materials are first crushed and ground to form a fine powder. The powder is then heated in a kiln to a peak temperature of 1,400-1,500° C., which results in sintering the powder which produces lumps or nodules referred to as clinkers. The heating process, among other things, drives off relatively large amounts of carbon dioxide. The production of one ton of portland cement releases one ton of carbon dioxide (CO2) into the atmosphere, accounting for 5 to 7 percent or more of the world's annual carbon dioxide emissions. The portland cement clinker is then ground to a fine powder with the addition of a small amount of calcium sulfate, usually derived from gypsum or anhydrite, as well as limestone powder in some cases. The finished powder is referred to as portland cement. Concrete or mortar made with portland cement sets relatively quickly and gains high compressive strength in a relatively short amount of time. Although great improvements have been made to the processes and efficiencies of portland cement manufacture, it is still a very expensive and highly polluting industrial process.
Fly ash is a by-product of the combustion of pulverized coal in electric power generation plants. When the pulverized coal is ignited in the combustion chamber, much of the carbon and volatile materials are burned off. However, some of the mineral impurities of clay, shale, feldspars, etc., are fused in suspension and carried out of the combustion chamber in the exhaust gases. As the exhaust gases cool, the fused materials solidify into spherical glassy particles called fly ash. When mixed with lime and water fly ash may form compounds similar to those formed from hydration of portland cement. Two classifications of fly ash are described in ASTM C 618, based upon composition, with their composition known to be related to the type of coal burned. Class F fly ash is normally produced from burning anthracite or bituminous coal that meets the applicable requirements. This Class of fly ash has pozzolanic properties and will have a minimum silicon dioxide plus aluminum oxide plus iron oxide of 70%. Class C fly ash is normally produced from subbituminous coal that meets the applicable requirements. This Class of fly ash, in addition to having pozzolanic properties, also has some cementitious properties and will have a minimum silicon dioxide plus aluminum oxide plus iron oxide content of 50%. Class C fly ash is used at dosages of 15% to 40% by mass of the cementitious materials in concrete, with the balance being portland cement. Class F fly ash is generally used at dosages of 15% to 40%, with the balance being portland cement. Use of fly ash in concrete in the U.S. is governed largely by ASTM Standard C 618. This standard prohibits the use of fly ash with too much residual carbon, which indicates that the coal was not burned thoroughly enough. Residual carbon impedes air entrainment and reduces the concrete's freeze-thaw resistance and may affect other properties as well. It is generally accepted that fly ash creates concrete with a higher compressive strength, but that this happens slowly over a longer period of time than concrete without fly ash. Fly ash-containing concretes also have to be managed differently as they cure, because they tend to cure and gain strength more slowly than mixes with more or a greater fraction of portland cement. Due to the slow compressive strength gain, concrete forms have to stay in place for many more days and perhaps weeks compared to concrete made with portland cement. Depending on the weather and ambient temperature, fly ash may not gain much strength at all in cold climates or in winter.
In the past, fly ash produced from coal combustion was simply entrained in flue gases and dispersed into the atmosphere. This created environmental and health concerns that prompted laws which have reduced fly ash emissions to less than 1 percent of ash production. Worldwide, more than 65% of fly ash produced from coal power stations is disposed of in landfills and ash ponds.
The recycling of fly ash has become an increasing concern in recent years due to increasing landfill costs and current interest in sustainable development. As of 2005, U.S. coal-fired power plants reported producing 71.1 million tons of fly ash, of which 29.1 million tons were reused in various applications. If the nearly 42 million tons of unused fly ash had been recycled, it would have reduced the need for approximately 27,500 acre·ft (33,900,000 m3) of landfill space. Other environmental benefits to recycling fly ash include reducing the demand for virgin materials that would need quarrying and substituting for materials that may be energy-intensive to create, such as portland cement.
As of 2006, about 125 million tons of coal-combustion byproducts, including fly ash, were produced in the U.S. each year, with about 43 percent of that amount used in commercial applications, according to the American Coal Ash Association. As of early 2008, the United States Environmental Protection Agency hoped that figure would increase to 50 percent as of 2011. More recently, there has been reduced interest in reusing fly ash. Of course, it is obvious that the more fly ash can be recycled, the better for the environment. Incorporation into concrete is one of the best way to utilize fly ash since once the concrete hardens the fly ash is encapsulated in the concrete and cannot leach out or escape into the environment. Furthermore, since there is such a large oversupply of fly ash, generally the cost is relatively low.
Fly ash can be used in concrete in two different ways: as a partial replacement for hydraulic cement or as filler. The first use takes advantage of the pozzolanic properties of fly ash, which, when it reacts with lime or calcium hydroxide, can enhance the strength of cementitious composites. However, fly ash is relatively inert and the increase in compressive strength can take up to 60 to 90 days or longer to materialize. Also, since fly ash is just a by-product from the power industry, the variable properties of fly ash have always been a major concern to the end users in the concrete industry, as variations in concrete properties at early and late ages may result.
The incorporation of fly ash in concrete improves workability and thereby reduces the water requirement with respect to conventional concrete. This is most beneficial where concrete is pumped into place. Among numerous other effects are reduced bleeding, reduced segregation, reduced permeability, increased plasticity, lowered heat of hydration, and increased setting times (ACI Committee 226, 1987, supra). Also, the slump is higher when fly ash is used (Ukita et al., 1989, SP-114, American Concrete Institute, Detroit, pp. 219-240). Comprehensive research demonstrated that high volume fly ash concretes showed higher long-term strength development, lower water and gas permeability, and higher chloride ion resistance in comparison with portland cement concretes without fly ash. See U.S. Pat. No. 6,818,058.
However, the prior art recognizes that the use of fly ash in concrete has many drawbacks. For example, the addition of fly ash to concrete results in a product with low air entrainment and low early strength development. As noted above, a critical drawback of the use of fly ash in concrete is that initially the fly ash significantly reduces the compressive strength of the concrete. Tests conducted by Ravindrarajah and Tam (1989, Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, SP-114, American Concrete Institute, Detroit, pp. 139-155) showed that the compressive strength of fly ash concrete at early ages are lower than those for the control concrete. Most of the reported studies tend to show a lower concrete strength due to the presence of fly ash when used as a partial replacement for portland cement; none has yet suggested a solution to actually enhance the property of concrete economically when using fly ash. Yet, for fly ash to be used as a partial replacement for cement, it must be comparable to cement in terms of strength contribution at a point useful in construction. As a practical matter, this means that the fly ash concrete must reach an acceptable compressive strength within days to be comparable to conventional or ordinary portland cement mixes.
Other widely used pozzolans are slag cement (also known as ground granulated blast furnace slag or GGBFS) and silica fume. Blast furnace slag is the non-metallic by-product of iron or steel production, generally consisting of silicon, calcium, aluminum, magnesium and oxygen. When iron is manufactured using a blast furnace, two products collect in the hearth—molten iron and slag. The slag floats to the top of the iron. The slag is skimmed off and fed to a granulator. In the granulator the molten slag is rapidly quenched with water. The resulting granules are essentially glassy, non-metallic silicates and aluminosilicates of calcium. The glass content of the slag generally determines its cementitious character or suitability for use in hydraulic cement. Generally, the higher the glass content the greater the cementitious properties of the slag. See U.S. Pat. No. 7,491,268. Ground slag suitable for use as hydraulic cement is described in ASTM C 989. For each metric ton of pig iron produced, there is approximately ⅓ of a metric ton of slag produced. In 2009 worldwide pig iron production was 1.211 billion tons. There was an estimated 400 million tons of slag produced. If slag is not granulated by quenching with water or steam and allowed to cool naturally, then it becomes an amorphous type aggregate. Aggregate made from slag is used for roadbeds and other filler application, but relatively little is used for the manufacture of slag cement due to relatively low demand for this waste material. In the past, amorphous slag was piled up close to steel plants creating a so called “brown fields.” Unfortunately, around the Great Lakes slag was even disposed of by dumping in the bottom of lakes. More recently, the U.S. has spent large sums of money to clean up these brown fields. Unfortunately, around the world relatively large amounts of amorphous slag sit in landfills close to iron furnace plants.
Concrete made with slag cement will have higher compressive and flexural strength growth over the lifetime of the concrete compared with conventional or ordinary portland cement concrete mixes. Slag cement improves the tensile strength capacity of concrete. Although when combined with relatively large amounts of portland cement slag cement sets faster than the fly ash, it is still slow to set and to gain strength when compared to conventional portland cement concrete. Hence, there is relatively low demand for the use of slag cement in concrete or mortar mixes. Therefore depending on the application, only a relatively small percentage of the portland cement is replaced with slag cement in concrete or mortar.
When water is added to hydraulic cement, a sequence of chemical reactions known collectively as “hydration” takes place. Hydration is an exothermic reaction, which means that the reaction produces heat. Thus, when concrete is initially mixed, it heats up due to a sequence of chemical reactions. But, in a relatively short amount of time, the heat produced decreases rapidly. The hydration reaction is temperature dependent. Therefore, the more heat, (i.e., higher ambient and/or concrete temperature), the faster the reaction; the less heat (i.e., colder), the slower the reaction. Thus, to cure concrete properly, two elements are necessary, appropriate temperature and availability of moisture. There is a direct relationship between the concrete temperature and the strength of the concrete in a given amount of time.
Maturity of concrete is measured as “equivalent age” and is given in temperature degrees×hours (either ° C.-Hrs or ° F.-Hrs). Maturity of concrete has became a useful tool in predicting the strength of concrete, particularly at ages earlier than 28 days and is related to the time and curing conditions, especially temperature. In this way, the maturity concept is also related to the rate of hydration and the rate of strength gain for a particular mix design.
Concrete slabs, walls, columns, various types of precast panels, precast structures, concrete pavers, artificial stone and other concrete structures, traditionally have been made by building a form. The forms are usually made from plywood, wood, metal and other structural members. Unhardened (i.e., plastic) concrete is poured into the space defined by opposed spaced form members or laying flat supported on the ground. Once the concrete develops sufficiently strength, the forms are removed leaving a concrete slab, walls, columns, precast panels and structures, pavers, artificial stone or other concrete structure or structural member; however, the concrete at this point is usually not completely cured. The unprotected concrete wall is then exposed to the elements during the remainder of the curing process. Since concrete is exposed to ambient temperatures, the initial heat of hydration is lost rather quickly to the surroundings, generally overnight. From that point on the concrete internal temperature follows very closely the ambient temperature. The exposure of the concrete to the elements, especially temperature variations, makes the curing of concrete, and the ultimate strength it can achieve, as unpredictable as the weather.
There is a disconnect between the type of forms in which concrete is cast and the curing to which it is subjected and the desired rate of rapid strength gain. Conventional concrete forms are designed to withstand a certain amount of pressure with the proper safety factor and be economical and easy to use. They seem to only serve the purpose of holding the plastic concrete mix in the desired form until it has generally hardened to around 2000 psi so that the forms can be stripped and reused. Since concrete forms are relatively expensive, concrete mixes are designed to set fast and achieve the necessary compressive strength to allow the forms to be stripped in approximately 1 to 3 days. Concrete curing, strength gain and internal concrete temperature have never been a concern for the concrete form manufacturers. Due to these constraints, and particularly the slow rate of strength gain of concrete or mortar made with fly ash or slag cement, the use of fly ash or slag cement in concrete has generally been limited to 20-30% of the cementitious material, with the balance being portland cement.
Concrete cures over a relatively long period of time. If it is desired for the concrete to cure more quickly or to have higher earlier strength, additives such as chemical accelerating admixtures can be added to the concrete mix. However, such additives are relatively expensive which significantly increases the cost of the concrete. If stronger concrete is required, the fraction of portland cement in the concrete is typically increased. However, portland cement is a major contributor to greenhouse gasses and is highly energy intensive to produce. Thus, portland cement and traditional concrete mixes are not very environmentally friendly.
Insulated concrete form systems are known in the prior art and typically are made from a plurality of modular form members. U.S. Pat. Nos. 5,497,592; 5,809,725; 6,668,503; 6,898,912 and 7,124,547 (the disclosures of which are all incorporated herein by reference) are exemplary of prior art modular insulated concrete form systems. Applicant's co-pending applications disclose insulated concrete form systems. See U.S. patent application Ser. Nos. 12/753,220 filed Apr. 2, 2010; 13/247,133 filed Sep. 28, 2011 and 13/247,256 filed Sep. 28, 2011 (the disclosures of which are both incorporated herein by reference in their entirety).
It is critically important in construction to have concrete or mortar that predictably achieves required performance characteristics; e.g., a minimum compressive strength within 1 to 3 days, to permit the forms to be stripped, and 7 to 14 days to place loads on the structure. Portland cement concrete achieves approximately 90%-95% of the ultimate compressive strength in the first 28 days. Therefore most concrete specifications are based on a 28-day strength. A corollary is that a construction or civil engineer must be able to predict the compressive strength of a concrete or mortar mixture after a given period of time. However, the prior art concrete or mortar mixtures that contain fly ash or slag cement lack predictability with respect to rate of compressive strength development and ultimate compressive strength, and generally have much lower early compressive strength than concrete or mortar mixtures that lack fly ash or slag cement. Therefore, there has been a disincentive to use fly ash or slag cement in such hardenable mixtures.
As previously noted, concrete quality is most commonly assessed based upon its 28-day strength, as measured through standard compression testing of concrete. Compression tests may be performed on concrete cast in the field, commonly tested as cylinders in North America, Australia, New Zealand, and France but as cubes elsewhere, including Great Britain and Germany. When cast in the field as cylinders, the concrete is placed in several lifts into a cylindrical mold with length-to-diameter ratio of 2.0, where the minimum cylinder diameter is at least three times the maximum aggregate size. The concrete is well-compacted typically through tamping, rodding, and/or use of vibration. After finishing, the cylinders are cured in a specified manner, often moist cured at 73.5±3.5° F. (23.0±2.0° C.) such as described by ASTM C192. Both of these common practices—the consolidation and curing processes—minimize variability and maximize strength development in concrete cylinders. Testing is also performed according to standard procedures, such as by ASTM C39, most commonly at 28 days but also at earlier and later ages when specified. Compressive strength measured on field-cast cylinders should be viewed as an assessment of the potential quality of the concrete and is not necessarily representative of the strength achieved in the same concrete cast as a structural element in the field. In the field, the compaction and curing conditions can be substantially different from those specified in ASTM C192, resulting in concrete with substantially lower strength than indicated from testing of cast cylinders.
When assessments of the strength or quality of concrete as-cast are of interest, compression testing can be performed on cylindrical concrete samples obtained from field structures. These concrete cores can be obtained by drilling into the hardened concrete with a diamond bit, as described in ASTM C42. Cores may be obtained in varying diameters and lengths, with an objective to obtain a length-to-diameter ratio of 2.0 and to achieve a diameter, which is at least three times the maximum aggregate size. However, it may not always be possible—due to reinforcement congestion, for example—to obtain cores meeting these specifications. As a result, the strength measured on these cores, then, may not reflect the actual strength of the as-cast concrete in the field. Other factors may also influence the strength measured in cores, generally resulting in a decrease in measured strength compared to actual strength. Such factors include the moisture content in the core and the uniformity or lack thereof in the moisture state, the state of stress in the structural element (i.e., in regions of tension, microcracking will decrease measured core strength) from where the core was obtained, the orientation of the core relative to the horizontal plane of placement (i.e., strengths may be lower near the top of a structure, due to bleeding or segregation), and damage induced in the core during cutting, extraction, and preparation (i.e., sawing to length, end grinding) for testing, among other factors. Thus, while core strengths are generally presumed to more accurately reflect the in-place concrete strength than standard-cured cast cylinders, the strength of the cores should not necessarily be presumed to be equivalent to the in-place concrete strength.
Predictions of concrete strength may also be made by applying the maturity concept, previously discussed. ASTM C918 describes how a maturity relationship can be developed for a particular mix design such that the strength can be anticipated based upon curing history, including temperature and age, and early measures of strength. However, it is important to recognize that accurate predictions can only be made if the concrete mix proportions and constituent materials, including type and composition of cementitious materials, aggregates, and any chemical admixtures, are exactly the same as those used to develop the maturity relationship. ASTM C192-07 provides some caution against predictions of strength based upon early age test results and maturity relationships: “Use of the results from this test method to predict specification compliance of strengths at later ages must be applied with caution because strength requirements in existing specifications and codes are not based upon early-age testing.” It is clear that the maturity relationship is complex and that predicted strengths should be viewed as only an indicator of in situ concrete strength.
Since both fly ash and slag cement are recycled materials, it would be desirable to produce a concrete composition that could employ relatively high amounts of these recycled materials. It would also be desirable to use reduced amounts of portland cement in concrete mixtures so as to reduce the amount of greenhouse gases that result from its manufacture. The challenge to the concrete industry has been to achieve these desired results without adversely affecting the compressive strength or other desirable properties of the finished concrete. It is believed that prior to the present invention, no one has been able to achieve these results. It would also be desirable to provide a concrete mix and a system for curing concrete that accelerates the maturity or equivalent age of concrete.